This proposal outlines the development of an antibody detection system which employs a bacteriorhodopsin-based photodetection device. The photodetector is prepared by depositing purple membranes isolated from Halobacterium halobium onto an electrode. Purple membranes contain bacteriorhodopsin which is an integral protein that, when stimulated by light, pumps protons across the lipid bilayer of the membrane. Purple membrane-based photodetectors have a wide variety of advantages over standard electronic photodetection systems including stability (they produce the same photoelectric response to light for years), compactness, sensitivity to light, low cost and the ability to elicit a differential response (i.e., respond to changes in light intensity with a constant light background). The proposed new photodetectors could provide advanced detection capabilities for discriminating between antibody isotypes, subgroups and immunospecificities. As commercial products, the proposed photodetection device can be incorporated into analytical instrumentations for diagnostic monitoring of patients at bedside and in emergency situations.